Convective orientational renewal in entangled polymers

The predictions of tube models in fast flows have recently been improved by incorporating convective constraint release (CCR). In this paper, we revisit CCR showing that, whenever chains are not stretched, an additional effect named convective conformation renewal (CCR2 to distinguish it from CCR) should also be considered. CCR2 consists in the loss of orientation of the unstretched chain due to flow-induced lengthening of tube segments. A simple quantitative model including both CCR and CCR2 is then developed, which reduces to double reptation in the linear limit. Although the model ignores fluctuations, the predicted flow curve is monotonic, and compares favourably with data for nearly monodisperse entangled polymers. For the case of a single relaxation time, typical of polymer-like micelles, a maximum in the flow curve still remains. (C) 2000 Elsevier Science B.V. All rights reserved.